Audio power amplifiers known to the art generally operate at a relatively low efficiency. To achieve greater efficiency, switching amplifiers have been employed, but these amplifiers still require a large audio frequency transformer if a transformer coupling to the load is employed.
U.S. Pat. No. 1,874,159, issued Aug. 30, 1932 is directed to an apparatus for amplifying an electric signal comprising a source of direct current, a load circuit, an interconnection between said source and said circuit including an inductive winding (transformer), a pair of electric valves (switches), each provided with a control element, means for energizing said control elements with a periodic potential of a frequency relatively high with respect to that of the signal and of an amplitude greater than that of the signal and means for modulating said high frequency potential with said signal.
U.S. Pat. No. 4,016,501, issued Apr. 5, 1977 is directed to a switching amplifier system which includes a pulse width modulator for converting a bipolar input signal into a train of unipolar pulses having durations which vary with the instaneous amplitude of the input signal. The pulses control the energizing of a series switch which supplies current to a switching bridge circuit. The switching arms of the bridge circuit are controlled by the polarity of the input signal. A path is provided for output current when the switch is open. The output current is filtered to remove fluctuations at the switching rate and applied to a load. It is stated that the amplified output voltage signal accurately follows the input signal, and the efficiency of conversion of DC power to AC power can approach 100 percent.
U.S. Pat. No. 3,585,517, issued June 15, 1971 is directed to a power amplifier which comprises pulse width modulating mean responsive to an input signal for producing an essentially square wave signal in which the width of the pulses is proportional to the instantaneous amplitude of the input or reference signal. Connected between the terminals of a source of supply voltage is an electric load device, a switch device such as a transistor and an inductor. The pulses periodically close the switches so that a portion of the voltage from the supply will appear across the inductor. When the switch opens upon termination of a pulse in the square wave, the energy stored in the inductor is discharged through the load device whereby unwanted voltage is stored in the inductor and discharged though the load device rather than being dissipated within the amplifier circuit elements.
U.S. Pat. No. 3,629,616, issued Dec. 21, 1971 is directed to an audio amplifier circuit wherein two 180.degree. out-of-phase audio signals are respectively applied to first and second comparators which compare the audio signals with a reference signal to produce two pulse-width modulated signals. The latter signals are applied to switching semiconductors connected in the two arms of a bridge which includes the transducer.
U.S. Pat. No. 2,990,516, issued June 27, 1961 is directed to a pulse width modulated amplifier and method which involves converting an input signal into a train of pulses time- or width-modulated in accordance with a function related to the amplitude of the signal, then amplifying said signal with a class B amplifier, and recovering an amplified signal corresponding to the original input signal.
U.S. Pat. No. 3,384,838, issued May 21, 1968, is directed to a phase reversible switching power amplifier wherein a low power pulse width modulator controls an electronic switch which is selectively connected to a positive and negative source of direct current. A signal to be amplified controls the width of the output pulses from the pulse width modulator and these pulses control the operation of the switch to connect to the positive direct current source when the signal is positive and to the negative direct current source when the signal is negative. The pulses resulting from the alternate connector to the direct current sources are then averaged to produce an output voltage which is linearly proportional to the input signal.
U.S. Pat. No. 4,092,610, issued May 30, 1978 is directed to a system for amplifying an input signal. The invention provides for the modulation of an input signal upon a carrier signal by a pulse duration modulator wherein the durations or widths of sequential pulses of the carrier signal are varied in accordance with variations in the amplitude of the input signal. The system includes a plurality of amplifier units which are sequentially operated for amplifying the modulated carrier signal and for coupling the modulated carrier signal to a load. Filter circuitry coupled between the plurality of amplifying units and the load provides a pass band which passes the spectrum of the input signal, the pass band being sufficiently narrow to exclude harmonics of the input signal and the carrier from the load.
A high efficiency audio amplifier has now been found which is not susceptible to the deficiencies of the prior art.